The hospital environment was designed for treatment of acute illnesses, and has evolved unsystematically over the past century based on financial, demographic, and political pressures. We have become expert at treating acute cardiac events in particular, but new evidence suggests that hospitalization itself may weaken our patients -- rendering them susceptible to a host of maladies while they recover. A recent study of 500,000 cardiac admissions found that 1 out of every 5 patients was rehospitalized within 30 days of their discharge. Surprisingly, only half of those rehospitalizations were for cardiac reasons. Because of new financial regulations in the Affordable Care Act that tie reimbursement to rehospitalization rates, hospitals are highly motivated to decrease these early rehospitalizations. A new model of posthospital syndrome (PHS) suggests that the stress of hospitalization itself confers increased risk of rehospitalization because it places patients in a transient state of generalized risk. Testing this model, and identifying hospital environment and patient factors that place patients at risk, are crucial for directing hospital efforts to reduce 30-day rehospitalizations. The first goal of this proposed research is to test the PHS model to explain the high risk of all-cause rehospitalization experienced by cardiac patients in the 30 days after they are discharged. The second goal is to identify modifiable hospital factors that contribute most to PHS, and by doing so detect targets for reducing early hospital readmissions. The third goal is to identify patient factors, such as personality and social support that may be associated with PHS, so that we can ultimately conduct risk stratification and improve patients' successful transition to independent post-hospital care. We plan to enroll a consecutive cohort of 1,000 cardiac patients who are admitted through the emergency department (ED), follow them throughout their hospitalization, and identify 30-day rehospitalizations. We will assess 4 in-hospital risk factors: stress using ecological momentary assessment (EMA); sleep and physical activity by actigraphy; and weight loss by electronic scale. We will also objectively assess hospital factors that may contribute to stress: stressful environmental factors (degree of ED crowding and length of stay [LOS]; excess noise and lighting throughout hospitalization). Further, we will measure patient factors that may influence the hospital experience, including personality, social support, cardiac severity, and physical and psychiatric comorbidities during and after hospitalization. Finally, we will test whether in-hospital stress (and secondarily, physical activity, sleep, or weight loss) contribute to 30-day rehospitalizations. More than 1 million cardiac patients are hospitalized in the US annually, and 20% are rehospitalized within 30 days. All of the factors that have been hypothesized to contribute to PHS are modifiable, but evidence of their association to rehospitalization is required to motivate change. This study would be the first to test whether change is warranted, and to provide targets for intervention. As such, the impact of this research is substantial.